Which of these numbers is prime? ${6,\ 51,\ 53,\ 65,\ 91}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 6 are 1, 2, 3, and 6. The factors of 51 are 1, 3, 17, and 51. The factors of 53 are 1 and 53. The factors of 65 are 1, 5, 13, and 65. The factors of 91 are 1, 7, 13, and 91. Thus, 53 is a prime number.